The field of the invention is that of surface wave components used in the field of electronics, in particular as RF or IF filters for the purpose of selecting frequency bands.
Field of the Invention
These components use the principle of acoustic propagation of surface waves, on the surface of a piezoelectric substrate. To ensure satisfactory surface acoustic wave propagation, the surface of the component must be protected against any contamination and any mechanical loading.
Furthermore, since the acoustic waves propagate on the surface of the piezoelectric substrate, this surface must be left free so as not to disturb the propagation of the waves, this constituting a further constraint at the level of the encapsulation packages.
The current technologies of encapsulation of surface wave filters rely on a two-part package, as illustrated in FIG. 1: a ceramic or organic base 01, and a ceramic, metal or organic cover 02, closure of which by welding or gluing ensures that the component is hermetically sealed while the necessary cavity is allowed for. In packages of this type, the surface wave devices (SAW) 03, can be assembled by gluing to the base. The electrical connections between the internal pads 011, 012 of the SAW and external pads 071 and 072 are provided for by metallized vias through the base 01.
FIG. 1 illustrates an example of the prior art, in which the electrical connections of the SAW with the exterior are of wire type. To provide better compactness, the xe2x80x9cflip chipxe2x80x9d type technique (the component being turned over) is currently employed. FIG. 2 illustrates an example of an encapsulated SAW according to the known art, which constitutes a variant of FIG. 1. The flip-chip type method is developed in detail in the U.S. Pat. No. 5,252,882. The SAW is transferred active face towards the base by way of metal beads known as xe2x80x9cbumpsxe2x80x9d which provides both for the electrical connection between the SAW and the base and the mechanical efficacy of this assembly. However, the limited number and the small size of these beads limit the mechanical efficacy of the assembly.
This is the reason that some have proposed the use of an encapsulation resin for enhancing the mechanical efficacy of these beads which provide for the electrical connections between the SAW and the base. Such resins are described in particular in European Patent EP 0 896 427 A2.
In this context, the invention proposes a new surface wave component comprising an SAW mounted active face towards a base and assembled to the said base with the aid of an anisotropic conductive adhesive.
The anisotropic conductive adhesive advantageously provides for an electrical connection function.
More precisely the subject of the invention is an encapsulated surface wave component comprising:
a surface wave device on the surface of a piezoelectric substrate, the so-called active surface, the said device comprising interdigital electrodes linked to conducting buses;
a base comprising electrodes linked to external conducting pads;
characterized in that: the active surface of the surface wave device is assembled to the base by way of a conductive adhesive which is anisotropic along an axis Z perpendicular to the plane of the surface wave device, at the level of the conducting buses opposite the electrodes of the base in such a way as to provide for electrical connections.
According to a variant of the invention, the anisotropic conducting adhesive can also be situated on the entire periphery of the surface wave device in such a way as to achieve the encapsulation of the said device.
The electrodes of the base have a structure suitable for compressing the anisotropic conductive adhesive locally, a necessary condition for squashing the conducting particles. According to the invention, it is therefore unnecessary to make bumps on the base or on the SAW. This structure comprises a succession of local thickenings allowing local compression, possibly of the order of a few tens of microns.
According to a preferred variant of the invention, the anisotropic conductive adhesive is a composite material comprising conducting particles and a binder material.
According to a variant of the invention, the electrodes of the base constitute barriers against the creeping of the anisotropic conductive adhesive towards the active zones of the surface wave device. Advantageously these electrodes can be in the form of a comb whose teeth are perpendicular to the direction of propagation of the acoustic waves of the surface wave device.